Magnetic recording medium

ABSTRACT

A magnetic recording medium comprising a support and a magnetic layer on said support. Said magnetic layer is a unitary magnetic layer comprising iron oxide powder having cobalt adsorbed on the surface thereof. The average grain size of said powder is not more than 0.5μ, the ratio of the major axis to the minor axis (acicular ratio) of said powder is from 1.3 to 4.5 and the coercive force of said powder is from 500 to 800 Oe.

This application is a continuation of application Ser. No. 740,509,filed June 3, 1985, now abandoned, which is a division of Ser. No.545,662, filed Oct. 26, 1983, now U.S. Pat. No. 4,572,866.

BACKGROUND OF THE INVENTION

This invention is concerned with improvements in magnetic recordingmedia and particularly with the magnetic recording media capable ofmaking magnetic recordings of high density and high resolution in thepresent video systems.

In order to improve the recording density of a magnetic recording mediumand to increase the sensitivity thereof, such a non-magnetic support aspolyethylene terephthalate, cellulose triacetate base or the like hasbeen coated with a magnetic layer comprising ferromagnetic powder andbinders both of which have been kneaded together. The ferromagneticpowder should have high residual magnetic flux density Br, ratio of theBr to saturation magnetic flux density Bm and coercive force Hc,respectively. The binders include vinyl chloridevinyl acetate copolymer,butyl acrylate-acrylonitrile copolymer, cellulose resin, polyurethaneresin and the like. To serve as the ferromagnetic powder, it isadvantageous from the viewpoint of magnetic characteristics such asBr/Bm, Br, Hc and the like to use a cobalt adsorbed iron oxide such asCo adsorbed γ-Fe₂ O₃ or metal magnetic powder. However, the conventionalmagnetic powder is an acicular crystal and if they are more finelygranulated to increase the resolution power, there can be obtained thosehaving the longer axis of 0.3 to 0.9μ and the shorter axis of 0.05 to0.01μ in length, and the ratio of such longer axis of the shorter axis,i.e., the acicular ratio is of the order of 5 to 10. Meanwhile, the Hcof these conventional metal magnetic powder is increased as they aregranulated to saturate the head of video deck to a value too great touse with the present video systems.

In magnetic powder of conventional Co adsorbed iron oxide having a highacicular ratio, such as Co-adsorbed γ-Fe₂ O₃, the saturationmagnetization σs is decreased as the powder is finely granulated, andthe high acicular ratio magnetic grains are easily broken to cause anuneven distribution of Hc in the magnetic layer, that is a cause ofimage transfer between laminated magnetic layers.

When using powder having a high acicular ratio, it is required tostrengthen the oriented magnetic field because of the necessity ofincreasing the degree of orientation. As a result, the grains are stucktogether, so that the surface of the magnetic layer is roughened. Suchpowder also tend to decrease in saturation magnetization σs and not tobe suitable for a high sensitivity and high density recording, as theybecome finer. It is a matter of course in the case of Co-adsorbed ironoxide that the decrease of saturation magnetization σs can be retardedif the amount of Co is increased. It is however undesired in practicaluse because other problems are derived therefrom. Namely, if the Coconcentration is too high thermal instability and pressure instabilityare caused with a resultant change of the magnetic characteristics.Further, Co is expensive in cost, that is not suitable in practicalapplication.

With respect to recorders of a coaxial circles type or a spiral typerecording system using ferromagnetic powder whose ratio of averagelonger axis to average shorter axis in length is within the scope of 1.2to 3.0, there has been disclosed in Japanese patent examined publicationNo. 32408/1982. However, this type of magnetic recording media, e.g.,the so-called floppy-disks, have so far been used, and the Hc of suchwell-known ferromagnetic powder is merely 20 to 400 Oe.

In the systems adopted in the present coaxial circles type or spiraltype magnetic recording media, the Hc has been prescribed in a limitedrange, and in the said publication there was also no disclosure toimprove the Hc of ferromagnetic powders in particular as well as toadopt to systems having a wider range of Hc. In the meanwhile,ferromagnetic powder having even higher Hc has been demanded in the newsystems of this type of field, however, γ-Fe₂ O₃ disclosed in the saidPublication cannot satisfy such demand. In addition, the conventionallyknown Co-containing γ-Fe₂ O₃ has had a problem in the thermalstabilization and has hardly been put into practical application.

In addition, Japanese Patent Publication Open to Public Inspection(hereinafter called Japanese Patent O.P.I. Publication) No. 98135/1982discloses a tape type magnetic recording medium bearing at least notless than two magnetic layers thereon. It discloses that the acicularratio of the ferromagnetic powder used in this magnetic recording mediumis higher than 1 and lower than 3. In such a magnetic tape, a video tapein particular, having not less than two magnetic layers as disclosed inthe described Publication, there are scattering of voices, brightness,picture noises and the like.

Further, dusts and dirts are apt to adhere to the coating surface of amagnetic layer to cause the so-called "Drop-out" phenomenon. This patentpublication does not describe any proper Hc but merely describe therecording media in which γ-Fe₂ O₃ having an extremely low coerciveforces of 280 to 330 Oe or Fe-powder having a normal Hc of 900 to 1050Oe.

In addition, Japanese Patent O.P.I. Publication No. 501053/1082discloses a magnetic recording element made of a metal oxide having ametallic dope such as, preferably, Co, having an acicular ratio of nothigher than 15 and preferably of 2 to 8 and the Hc of not lower than 450Oe and preferably of not lower than 650 Oe. However, there is a problemof thermal stability in such magnetic recording media using a metaloxide of a metallic dope such as Co dope γ-Fe₂ O₃.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a high sensitivity and highdensity magnetic recording medium.

The inventors have various studied magnetic recording media andparticularly high density and high resolution magnetic recording mediato be applied to the present video systems, and have been able toconstitute a high density and high output magnetic recording mediumwhich has not ever been obtainable in the prior art by lowering theacicular ratio of Co-adsorbed iron oxide or metallic magnetic powder andby giving a proper Hc to the magnetic powder finely granulated.

This invention is to eliminate the disadvantages that disturb theapplication of Co-adsorbed iron oxide magnetic powder or metallicmagnetic powder to the present video system by increasing Hc of themetallic magnetic powder or by decreasing the saturation magnetizationσs of adsorbed γ-Fe₂ O₃. Namely, the coercive force Hc is within therange of 550 to 800 Oe and the saturation magnetization Br is notdecreased.

This invention is to provide a magnetic recording medium bearing asupport base coated thereon with a magnetic layer containingferromagnetic powder and the magnetic recording medium characterized inthat the ferromagnetic powder is one selected from magnetic metallicpowder and Co-adsorbed iron oxide magnetic powder whose grain size isnot larger than 0.5μ and the acicular ratio Ra and coercive force Hc arein the following relations, respectively;

    1.3≦Ra≦4.5

    550≦Hc≦800 Oersted

There may be given the following alloys as the examples of magneticmetallic powder defined as those comprising a metal or a metal alloywithout containing a metal compound such as γ-iron oxide: Fe, Ni, Co,Cr, Fe-Co-Ni, Fe-Ni-Zn, Fe-Mn-Zn, Fe-Al, MnBi, Fe-Al-P, Fe-Co-Ni-Cr,Fe-Co-Ni-P, Fe-Ni-Mn, Ni-Co, Fe-Ni, Co-Ni-P, Co-P, Fe-Ni-Cr-P,Fe-Ni-Cr-Zn, Co-P and inter alia, it is preferred to use a magnetic ironpowder of which Fe is not less than 60% by weight and preferably notless than 80% by weight. As a small amount of additives, it is allowableto use, for example, Mg, Mn, Si, the other various elements and thecompounds thereof. The grain size of the described ferromagnetic powderis not larger than 0.5 μm (e.g., 0.05-0.5 μm) and more preferably, notlarger than 0.3 μm (e.g. 0.1-0.3 μm) for the purpose of high densityrecording.

As a Co-adsorbed iron oxide, Co-adsorbed γ-Fe₂ O₃ is preferred amongthem. The amount of Co adsorbed is preferably 0.5 to 30% by weight andmore preferably 2.5 to 30% by weight.

If the amount of Co adsorbed is less than 0.5% by weight, the Hc is low,and if more than 30% by weight, thermal instability will be caused causeand the head of the video deck is also apt to be worn out.

As the materials of a support base (hereinafter called a support), theremay be used polyethylene terephthalate; a polyester such aspolyethylene-2,6-naphthalate and the like; a polyolefin such aspolypropylene and the like; a cellulose derivative such as cellulosetriacetate, cellulose diacetate and the like; a plastic such aspolycarbonate and the like; a non-magnetic metal such as Al, Zn and thelike; and a ceramic.

The thickness of the supports is of the order of about 3 to 100 μm andpreferably 5 to 50 μm in the form of film or sheet, and of the order of30 to 10 mm in the form of disk or card. In the case of a drum, the formthereof is to be cylindrical, and the shape will be determined inaccordance with the recorder to be used.

The described supports may also be back-coated on the surface oppositeto the surface provided thereon with a magnetic layer with the purposesof preventing an electric charge, an image transfer and the like.

There are described such back-coating in the following patentspecifications; U.S. Pat. Nos. 2,804,401, 3,293,066, 3,617,378,3,062,676, 3,734,772, 3,476,596, 2,643,048, 2,803,556, 2,887,462,2,923,642, 2,997,451 3,007,892, 3,041,196, 3,115,420, 3,166,688 and thelike.

The form of the supports may be anyone of tapes, sheets, cards, disks,drums and the like. Various materials may be selected according to theform to meet a necessity.

As the binders, conventionally well-known thermoplastic resins,thermosetting resins, reactive resins, electron-beam irradiated settingresins or mixtures thereof may be used.

As the thermoplastic resins, there are used those having a softeningtemperature of not higher than 150° C., an average molecular weight of10,000 to 200,000 and a degree of polymerization of about the order of200 to 2,000; for example, vinyl chloride-vinyl acetate copolymer, vinylchloride-vinylidene chloride copolymer, vinyl chloride-acrylonitrilecopolymer, an ester acrylate-acrylonitrile copolymer, an esteracrylate-vinylidene chloride copolymer, an ester acrylate-styrenecopolymer, an ester methacrylate-acrylonitrile copolymer, an estermethacrylate-vinylidene chloride copolymer, an estermethacrylate-styrene copolymer, urethane elastomer, polyvinyl fluoride,vinylidene chloride-acrylonitrile copolymer, acrylonitrile-butadienecopolyer, polyamide resin, polyvinyl butyral, a cellulose derivativesuch as cellulose acetate butyrate, cellulose diacetate, cellulosetriacetate, cellulose propionate, nitrocellulose and the like,styrene-butadiene copolymer, polyester resin, chlorovinyl ether-an esteracrylate copolymer, amino resin, various synthetic rubber types ofthermoplastic resin, and mixtures thereof, and the like.

These resins are described in Japanese patent examined publication Nos.6877/1962, 12528/1964, 19282/1964, 5349/1965, 20907/1965, 9463/1966,14059/1966, 16985/1966, 6428/1967, 11621/1967, 4623/1968, 15206/1968,2889/1969, 17947/1969, 18232/1969, 14020/1970, 14500/1970, 18573/1972,22063/1972, 22064/1972, 22068/1972, 22069/1972, 22070/1972 and27886/1973; and U.S. Pat. Nos. 3,144,352, 3,419,420, 3,499,789 and3,713,887.

The molecular weight of such thermoplastic resins or reaction typeresins are not more than 200,000 when they are in the state of coatingliquid and are infinite after a reaction such as a condensation reactionor an addition reaction and after thay are coated and dried. Inter alia,it is preferred that such resins should not soften or fuse until theyare thermally decomposed. More concretely, for example, they are phenolresin, polyurethane setting type resin, urea resin, melamine resin,alkyd resin, silicone resin, acryl type reactive resin, mixture of amacromolecular polyester resin and isocyanate prepolymer, mixture of amethacrylate copolymer and diisocyanate prepolymer, mixture of polyesterpolyol and polyisocyanate, mixture of urea formaldehyde resin/a lowmolecular glycol/a macromolecular diol/triphenyl methane isocyanate,polyamine resin, and mixtures thereof.

These resins are described in Japanese patent examined publication Nos.8103/1964, 9779/1965, 7192/1966, 8016/1966, 14275/1966, 18179/1967,12081/1968, 28023/1969, 14501/1970, 24902/1970, 13103/1971, 22067/1972,22072/1972, 22073/1972, 28045/1972, 28048/1972, and 28922/1972, and U.S.Pat. Nos. 3,144,353, 3,320,090, 3,437,510, 3,597,273, 3,781,210, and3,781,211.

Electron-beam irradiation setting type resins include an unsaturatedprepolymer such as maleic anhydride type, urethane acryl type, polyesteracryl type, polyether acryl type, polyurethane acryl type, polyamideacryl type and the like; and multifunctional monomers include those ofether acryl type, urethane acryl type, phosphate acryl type, aryl type,hydrocarbon type and the like.

These binders are used independently or in combination, and besides,additives may be added if necessary.

The mixture proportion of ferromagnetic powder of the invention and thebinders is within the range of 5 to 400 parts by weight, and preferably10 to 200 parts by weight of the binders to 100 parts by weight of theferromagnetic powders.

If the binders are used in excess thereof to produce a magneticrecording medium, the recording density of the recording medium islowered. If used in too small an amount, the strength of a magneticlayer is weakened so that an unfavorable phenomenon such as thedeterioration of durability, shedding and the like will occur.

To improve the ferromagnetic recording media relating to the inventionto be more durable, various hardening agents such as a polyisocyanate,may be contained in a magnetic layer.

Such polyisocyanates include the addition product of diisocyanate andtervalent polyol, or a decarboxylated compound of diisocyanatepentamer/3 mole of diisocyanate/water.

The examples thereof include the addition product of 3 mole of tolylenediisocyanate/1 mole of trimethylol propane the, addition product of 3mole of methaxylylene diisocyanate/1 mole of trimethylol propane,tolylenediisocyanate pentamer, pentamer comprising 3 mole of tolylenediisocyanate/2 mole of hexamethylene diisocyanate, the decarboxylatedproduct obtainable by making 3 mole of hexamethylene diisocyanate reactwith 1 mole of water, and the like. These may readily be prepared in anindustrial scale.

Magnetic layers containing the ferromagnetic powder of the invention,the described binders, and the hardening agents, may also contain suchadditives as a dispersing agent, coupling agent, lubricant, abrasives,antistatic agent and the like.

As for the dispersing agents, there may be used a fatty acid having 8 to18 carbon atoms, such as caprylic acid, capric acid, lauric acid,myristic acid, palmitic acid, stearic acid, oleic acid, elaidic acid,linolic acid, linolenic acid and the like, which are represented byR--OOOH, wherein R represents a saturated or unsaturated hydrocarbongroup having 7 to 17 carbon atoms; a metallic soap comprising an alkalimetal of the described fatty acide such as Li, Na, K, etc., or analkaline earth metal such as Mg, Ca, Ba, etc. Besides, a higher alcoholhaving not less than 12 carbon atoms, a sulfate and the like can also beused. These dispersing agents may be used independently or incombination. These dispersing agents are to be added within the range of1 to 20 parts by weight to 100 parts by weight of the described powder.

These dispersing agents are described in Japanese patent examinedpublication Nos. 28369/1964, 17945/1969, and 1500/1973, and U.S. Pat.Nos. 3,587,993, and 3,470,021, and the like.

The coupling agents for excellently dispersing the ferromagnetic powderwhich were finely powdered as mentioned above, includes isopropyltriisostearoyl titanate, isopropyl tridecyl benzene sulfonyl titanate,isopropyl tri(dioctyl pyrophosphate)titanate, tetraisopropyl bis(bioctylphosphite)titanate, tetraoctyl bis(ditridecyl phosphite)titanate,tetra(2,2-diallyloxymethyl-1-butyl)bis(di-tridecyl phosphite) titanate,bis(dioctyl pyrophosphate)oxyacetate titanate, bis (dioctylpyrophosphate)ethylene titanate, and the like. The ferromagnetic powdermay be treated in advance or may also be added into a magnetic paint.

The lubricants, which can used include silicone oil, carbon black,graphite, carbon block graft polymer, molybdenum disulfide, tungstendisulfide, a fatty acid ester, i.e., a wax, comprising a monobasic fattyacid having 12 to 16 carbon atoms and a monovalent alcohol whose carbonatom number is to have 21 to 23 if adding the carbon atom number of thedescribed monobasic fatty acid to the carbon atom number thereof. Theselubricants may be added within the range of 0.2 to 20 parts by weightthereof to 100 parts by weight of the described powder. These lubricantsare described in Japanese patent examined publications Nos. 23889/1968,81543/1968 and the like; U.S. Pat. Nos. 3,470,021, 3,492,235, 3,497,411,3,523,086, 3,625,760, 3,630,772, 3,634,253, 3,642,539, and 3,687,725;IBM Technical Disclosure Bulletin, vol. 9, No. 7, p. 779 (Dec. 1966);Electronik 1961, No. 12, p. 380; and the like.

The abrasives which can be used include fused alumina, silicon carbide,chromium oxide, corundum, artificial corundum, diamond, artificialdiamond, garnet, emery mainly comprising corundum and magnetite, and thelike. They are used in the average particle size of 0.05 to 5μ and morepreferably, 0.1 to 2μ. They are added within the range of 1 to 20 partsby weight to 100 parts by weight of the powder. These abrasives aredescribed in Japanese Patent O.P.I. Publication No. 115510/1974; U.S.Pat. Nos. 3,007,807, 3,041,196, and 3,687,725; British Pat. No.1,145,349; and West German Patent (DT-PS) No. 853,211.

The antistatic agents, which can be used include an electroconductivepowder such as graphite, carbon black, tin oxide-antimonium oxide typecompounds, tin oxide-titanium oxide-antimonium oxide type compounds,carbon black graft polymer and the like; natural surface active agentssuch as saponin; nonionic surface active agents such as alkylene oxidetype, glycerol type, glycidol type or the like; cationic surface activeagents such as higher alkylamine type, quaternary ammonium salt type,heterocyclic type of pyridine or the like, cationic surface activeagents such as phosphonium or sulfonium type, or the like; anionicsurface active agents containing such an acid radical as carbonic acid,sulfonic acid, phosphoric acid, sulfate, phosphate or the like;amphoteric surface active agents such as amino acid type, aminosulfonicacid type, sulfate or phosphoate of aminoalcohol; and the like.

The surface active agents capable of serving as the abovementionedantistatic agents are described in U.S. Pat. Nos. 2,271,623, 2,240,472,2,288,226, 2,676,122, 2,676,924, 2,676,975, 2,691,566, 2,727,860,2,730,498, 2,742,379, 2,739,891, 3,068,101, 3,158,484, 3,201,2533,210,191, 3,294,540, 3,415,649, 3,441,413, 3,442,654, 3,475,174, and3,545,974; West German Pat. (OLS) No. 1,942,665; British Pat. Nos.1,077,317, and 1,198,450; and the like patents; Ryohei Oda, "Synthesisand Application of Surface Active Agents", Maki Bookstore, Japan, 1964;A. M. Schwartz & J. W. Perry, "Surface Active Agents", IntersciencePublication, Inc., 1958; J. P. Sisley, "Encyclopedia of Surface ActiveAgents", vol. 2, Chemical Publishing Co., 1964; "A Handbook of SurfaceActive Agents", 6th ed., Sangyo Tosho K.K., Dec. 20, 1966; and the like.

These surface active agents may be added independently or incombination. these surface active agents are to be used as antistaticagents, and there are also some instances where they are used with theother purposes of, for example, dispersing, improving the magneticcharacteristics and lubricity, and serving as a coating aid.

The solvents for magnetic coatings to be used in a magnetic coatingprocess, include solvents of ketone type such as acetone, methylethylketone, methylisobutyl ketone, cyclohexanone or the like; those ofalcohol type such as that of methanol, ethanol, propanol, butanol or thelike; those of ester type such as that of methyl acetate, ethyl acetate,butyl acetate, ethyl lactate, glycol acetate monoethyl ether, or thelike; those of glycol ether type such as that of glycol dimethyl ether,glycol monoethyl ether, dioxane, or the like; those of aromatichydrocarbon type such as that of benzene, toluene, xylene or the like;those of halogenated hydrocarbon type such as that of methylenechloride, ethylene chloride, carbon tetrachloride, chloroform,dichlorobenzene or the like.

Ferromagnetic powder of the invention, binders, dispersing agents,lubricants, abrasives, antistatic agents, solvents and the like aremixedly kneaded to prepare magnetic coatings.

When mixedly kneading, the abovementioned ferromagnetic powder and eachof the above components are put into a kneading machine altogether atthe same time or one by one in order. For instance, there is a processfor preparing a magnetic paint, wherein the above ferromagnetic powderis added first to a solution containing a dispersing agent and kneadedfor a prescribed period. Then, each of the other components is added andkneaded, and thus a magnetic coating is prepared.

When mixedly kneading and dispersing, various types of kneading machinesmay be used. For instance, they may be a double roller mill, tripleroller mill, ball mill, pebble mill, sand grinder, Szegvari attriter,high-speed impeller dispersing machine, high-speed stone mill,high-velocity impact mill, disper-kneader, high speed mixer,homogenizer, ultrasonic dispersing machine and the like.

The technique of kneading-dispersion is described in T. C. Patton,"Paint Flow of Pigment Dispersion", John Wiley & Sons, 1964, and in U.S.Pat. Nos. 2,581,414 and 2,855,156.

As a coating method for forming a magnetic layer by coating theabove-mentioned magnetic coating on a support, there can be utilizedair-knife coating, air-doctor coating, blade coating, reverse rollcoating, gravure coating, transfer roll coating, kiss coating, castcoating, spray coating, squeeze coating and the like. Other methods mayalso be used and the details of such methods are described in "CoatingEngineering", Asakura Bookstore, 1971.

A magnetic layer which was coated on a support in such a coating methodas mentioned above is treated, if necessary, to orient the ferromagneticpowder of the invention contained in the layer, and then the magneticlayer thus formed is dried. If also necessary, the surface of the layeris processed to be smooth and is cut into a desired size to prepare amagnetic recording medium of the invention.

It is desired in this case that the oriented magnetic field is A.C. orD.C. of the order of about 500 to 3500 Gauss; and the drying temperatureis of the order of about 50° to 100° C.

The following apply to the magnetic recording media of the presentinvention:

(1) The thickness of a magnetic layer can be thinned to, for example,1.2μ to 3.8μ; Inside of a magnetic layer can be prevented from forming adiamagnetic field; and thus a higher density recording can be performedas compared with those of conventional magnetic recording media; becauseof the finely pulverized ferromagnetic powders.

(2) The distribution of Hc caused by scattering of fragments offerromagnetic powder is decreased and thus a less transferable magneticrecording medium can be prepared; because the acicular ratio is low sothat the ferromagnetic powders are rarely to be broken when they aredispersed.

(3) Oriented magnetic field of high strength is not needed to increasethe degree of orientation; and the roughness caused thereby on thesurface of the magnetic layer can be reduced; because of the lowacicular ratio of the ferromagnetic powder.

(4) Not only the signal-to-noise ratio (S/N) is excellent because of thefinely pulverized ferromagnetic powder, but also thebinder-to-ferromagnetic powder proportion can suitably be adjustedbecause of the high saturation magnetization σs.

(5) A high output recording medium can be prepared.

(6) Quantity and quality of binders having a strong physical property ofthe coating can be selected.

(7) Especially with a magnetic recording medium relating to theinvention in which ferromagnetic metal powder is used, thepreservability against a change on standing of thus prepared magneticrecording media can be excellent, because the tip (which is apt to bestained) of a highly active acicular crystal can be smaller than thoseof conventional ferromagnetic metal powder having an acicular ratiowhich is high.

(8) Particularly when the ferromagnetic powder a magnetic metal powder,it is possible to use them in the present video systems because theacicular ratio thereof is as low as 1.3 to 4.5, and Hc is 550 to 800 Oeand preferably 580 to 700 Oe; and a high output magnetic recordingmedium can be obtained because the high saturation magnetization σswhich is originally displayed by magnetic metal powder can be utilized,and

(9) Unexpectedly, the durability of an image in the still is remarkablyexcellent in comparison with the conventional ones.

In the invention, "Co-adsorbed iron oxide" is defined as Co that isadsorbed to the surface of iron oxide which has been prepared in theprocess wherein Geothite is changed into γ-iron oxide, for example, in adehydration process and successively in a reduction process and anoxidation reaction, and then the iron oxide is mixed into an alkalinesolution for example to add CoSO₄ and FeSO₄ (or CoSO₄ only) thereto.Accordingly, the case of "Co-adsorbed" of the invention is differentfrom the other cases such as the case of Co-doped, Co-singlycoprecipitated or the like from the viewpoint of Co-contained, and isalso different from the case of the conventional "Co-contained ironoxide". For example, the case of Co-adsorbed of the invention is furtherdifferent from the case of "Co-doped iron oxide" described in JapanesePatent O.P.I. Publication No. 501053/1982 and is still further differentfrom the case of iron oxide coprecipitated with Co-ion at the finalstage of the reaction of the growth of Geothite described in Example 3of Japanese Patent O.P.I. Publication No. 98135/1982. Comparing withthese publicly known "Co-contained iron oxide", "Co-adsorbed iron oxide"relating to the invention is superior thereto in thermal stability, andthus an excellent magnetic recording medium can be provided as mentionedlater.

Further, the abovementioned "fine pulverization" means that the grainsize of magnetic powder is to make smaller, and in the invention thegrain size thereof is not larger than 0.5μ and preferably not largerthan 0.3μ for further higher density recording.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The following are the detailed description of the invention withreference to the control examples and the examples of the invention. Theword, a "part" appeared in the examples means a "part by weight".

CONTROL EXAMPLE 1

    ______________________________________                                        Magnetic powders of an Fe alloy                                                                         75    parts                                         of which the acicular ratio is 8                                              and the grain size is 0.4μ                                                 Vinyl chloride-vinyl acetate                                                                            5     parts                                         copolymer, "VAGH", mfd. by Union                                              Carbide Co.                                                                   Polyurethane resin, "Estane-5701"                                                                       9     parts                                         mfd. by Goodrich Co.                                                          Methylethyl ketone        70    parts                                         Toluene                   60    parts                                         Cyclohexane               5     parts                                         Tetraisopropyl bis(dioctyl phosphite)titanate                                                           1.5   parts                                         ______________________________________                                    

The abovementioned composition was thoroughly mixed and dispersed, andthen five parts of "Colonate L", (a polyisocyanate solution, mfd. byNippon polyurethane Co.) were added to the resulting dispersed mixture,and thus a magnetic paint was preared by uniformly mixing. The obtainedmagnetic paint was coated onto one side of polyethylene terephthalatebase of 12μ in thickness so that the coating thickness can be 5μ, with amagnetic field of 2000 Gauss being applied thereto and drying.

The obtained sample of wide width was treated in a supercalender processand was then slitted into 12.65 mm each to obtain video-tapes as thecontrol sample No. I-(0).

EXAMPLES 1-5

    ______________________________________                                        Magnetic powder of Fe metals of                                                                        75    parts                                          which the acicular ratio is 3 and                                             the grain size is 0.3μ                                                     VAGH                     5     parts                                          Polyurethane resin, "Estane-5701"                                                                      9     parts                                          mfd. by Goodrich Co.                                                          Methylethyl ketone       70    parts                                          Toluene                  60    parts                                          Cyclohexane              5     parts                                          Tetraisopropyl bis       1.5   parts                                          (dioctyl phosphite)titanate                                                   ______________________________________                                    

The abovementioned composition was thoroughly mixed and dispersed, andthen five parts of "Colonate L" were added to the resulting dispersedmixture, and thus a magnetic paint was prepared by uniformly mixing. Theobtained magnetic paint was coated onto one side of polyethyleneterephthalate base 12μ thick so that the coating thickness can be 5μ,with a magnetic field of 2000 Gauss being applied thereto and drying.

The obtained sample of wide width was treated in a supercalender processand was then slitted into 12.65 mm each to obtain video-tapes as thecontrol sample No. I-(1).

As the iron alloy magnetic powder in the abovementioned composition,Fe-Co-Ni magnetic powder in the configurations of the acicular ratiosand the grain sizes of 4 and 0.2μ; 4 and 0.3μ; 2.5 and 0.3μ; 3.5 and0.3μ; and 4.2 and 0.2μ; respectively, are used in place of those of theacicular ratio of 3 and the grain size of 0.3μ. They were thoroughlymixedly dispersed by means of a ball mill as were done similarly in theabove case, and five parts of "Colonate L" were added thereto and werethen uniformly mixed up. The obtained magnetic paints were each coatedonto one side of respective polyethylene terephthalate bases so that thecoating thickness can be 5μ with applying magnetic fields thereto anddrying. Further, super-calender processes were applied respectively toprepare video tapes of 12.65 mm in width. Thus prepared video tapes werecalled Samples I-(1), I-(2), I-(3), I-(4), and I-(5), respectively.

Table-1 below shows the measurement results of the performance of thetapes of thus obtained Control Sample I-(0) and Samples I-(1), I-(2),I-(3), I-(4) and I-(5) obtained in Examples 1-5.

                  TABLE-1                                                         ______________________________________                                        Performance                                                                               Grain                                                                         size                                                              Hc    RF    Chrom.   Gloss                                                    Sample                                                                              Ra    (μ)   (Oersted)                                                                            Output S/N(dB)                                                                              %                                   ______________________________________                                        I-(0) 8     0.4      1050   0      0      148                                 I-(1) 3     0.3      580    +0.5   +0.6   153                                 I-(2) 4     0.2      700    +1.0   +2.2   168                                 I-(3) 2.5   0.3      650    +1.0   +1.2   158                                 I-(4) 3.5   0.3      600    +0.7   +0.7   157                                 I-(5) 4.2   0.2      750    +2.3   +2.5   160                                 ______________________________________                                    

In the table,

(a) Radio frequency output

RF output were measured at 4 MHz by means of a video deck for measuringRF output, and the results thereof are shown as the relative values tothat of the output of Control Sample I-(0) that is taken as 0.

(b) Chrominance S/N

Chrominance signal of 3.58 MHz was set at 0.714 Vp-p to make a picturerecording by putting the chrominance signal into a brightness modulationsignal, and the chrominance signal was solely taken out by reproducingthe recorded picture to obtain the ratios of the effective values (S) tothe noise levels (N) from which the level of the chrominance signal wasremoved. The ratios are exhibited by the unit of dB.

(c) Degrees of gloss (which were measured by Glossmeter GL-26D mfd. byMurakami Color Research Labs., Japan).

The degrees of gloss are expressed as the relative values in the term ofpercentage to which the degree of gloss obtained from a black tile madecontrol sample is provided as 100% when they are reflected by light withthe angle of 60° to the surfaces thereof. The higher the values are, thehigher the outputs are.

According to the results shown in Table-1, there shows in the case ofFe-type magnetic powder relating to the invention that Hc is apparentlybetween 580 and 750 Oe when the acicular ratio is between 1.3 and 4.5,that is, the Hc thereof are far smaller than those (of the order of900-1200 Oe) of the ordinary metal type ferromagnetic powder. When theacicular ratio becomes not greater than 1.3 and the powder becomesnearly globular, the form magnetic anisotropy becomes smaller and themagnetic anisotropy for tapes is missed out. When the acicular ratio isnot lower than 4.5, the magnetic powder is apt to be broken and the Hcis distributed, so that the magnetic powder will become unsuitable for ahigh density recording.

CONTROL EXAMPLE 2

    ______________________________________                                        Co-adsorbed-γ-Fe.sub.2 O.sub.3                                                                    75    parts                                         (Acicular ratio: 11; Grain length: 0.3μ)                                   VAGH                      5     parts                                         "Estane 5701"             9     parts                                         mfd. by Goodrich Co.                                                          Methylethyl ketone        70    parts                                         Toluene                   60    parts                                         Cyclohexanone             5     parts                                         Isopropyl tris(dioctyl    1.5   parts                                         pyrophosphate)titanate                                                        ______________________________________                                    

The abovementioned composition was thoroughly mixed and dispersed, andthen five parts of "Colonate L" were added to the resulting dispersedmixture, and thus a magnetic paint was prepared by uniformly mixing. Theobtained magnetic paint was coated onto one side of polyethyleneterephthalate base 12μ thick so that the coating thickness can be 5μ,with a magnetic field of 2000 Gauss being applied thereto and drying.

The obtained sample of wide width was treated in a supercalender processand was then slitted into 12.7 mm to obtain video-tapes and the controlsample No. II-(0).

    ______________________________________                                        Examples 6-10                                                                 Co-adsorbed-Fe.sub.2 O.sub.3                                                                         75    parts                                            VAGH                   5     parts                                            "Estane 5701"          9     parts                                            mfd. by Goodrich Co.                                                          Methylethyl ketone     70    parts                                            Cyclohexanone          5     parts                                            Isopropyl tris(dioctyl 1.5   parts                                            pyrophosphate)titanate                                                        ______________________________________                                    

The abovementioned compositions of which the acicular ratios ofCo-adsorbed-γ-Fe₂ O₃ powder were 4.2, 4, 3.5, 3 and 2.5, respectively,were thoroughly mixedly dispersed by a ball mill and five parts of"colonate L" were added thereto and were then uniformly mixed up. Theobtained magnetic paints were each coated onto one side of respectivepolyethylene terephthalate bases with applying magnetic fields of 2000Gauss, and were then dried up so that the thickness can be 5μ each. Theobtained samples of wide width were treated in supercalender process andwere then slitted into 12.65 mm in width to obtain video-tapes as thecontrol Samples No. II-(1), II-(2), II-(3), II-(4) and II-5,respectively.

Table-2 below exhibits the measurement results of the tape performancei.e., Hc, RF outputs in dB, chrom S/N ratios and the degrees of gloss ofSample Nos. II-(1), II-(2), II-(3), II-(4) and II-(5) and Control SampleNo. II-(0).

                  TABLE-2                                                         ______________________________________                                        Performance                                                                                Grain                                                                         size                                                             Hc    RF     Chroma  Gloss                                                    Sample                                                                              Ra     (μ)  (Oersted)                                                                            Output S/N(db)                                                                              %                                   ______________________________________                                        II-(0)                                                                              11     0.3     680    0      0      150                                 II-(1)                                                                              4.2    0.2     550    +0.9   +0.8   163                                 II-(2)                                                                              4      0.3     580    +0.9   +0.5   148                                 II-(3)                                                                              3.5    0.3     650    +1.3   +0.8   168                                 II-(4)                                                                              3      0.3     630    +1.5   +0.8   168                                 II-(5)                                                                              2.5    0.4     700    +2.0   +1.2   170                                 ______________________________________                                    

From the contents of Table-2, even in the case of Co-adsorbed-γ-Fe₂ O₃magnetic powder, it is needless to say that the form anisotropy becomessmaller when the acicular ratio is not higher than 1.3, and it is foundthat they are not preferred to be a magnetic recording medium becausethe magnetic powder is apt to be broken and the Hc is distributed tocause an image transfer trouble, when the acicular ratio thereof is notlower than 4.5.

The above pointed out characteristics are common not only to Fe typealloy magnetic powder and Co-absorbed-γ-Fe₂ O₃ magnetic powder but alsoto the other metal type ferromagnetic powder and the other Co-adsorbediron oxide.

EXAMPLE 11

With the purpose of investigating the durability of still images ofSample Video Tapes II-(2) and II-(4) which were obtained in Examples 7and 9, the changes on standing of the still images thereof were measuredby means of a video deck, NV-6200, mfd by Matsushita Electric IndustrialCo., Ltd., Japan. On the other hand, the measurements were made on theSample Video Tape II-(0) obtained, as a control sample, in ControlExample-2. The results thereof are shown in Table-3 below.

                  TABLE-3                                                         ______________________________________                                                       Characteristics                                                Sample No.      Results of Measurements                                       ______________________________________                                        (Invention) II-(2)                                                                            Excellent still images were                                                   displayed even after a lapse                                                  of over four hours.                                           (Invention) II-(4)                                                                            Excellent still images were                                                   displayed even after a lapse                                                  of over four hours.                                           (Control) II-(0)                                                                              Images almost disappeared                                                     after a lapse of one hour.                                    ______________________________________                                    

We claim:
 1. A magnetic recording medium having improved thermalstability comprising a support and a magnetic layer on said support,saidmagnetic layer being a unitary magnetic layer comprising iron oxidepowder having cobalt adsorbed on the surface thereof, the average grainsize of said iron oxide powder being not more than 0.5μ, the ratio ofthe major axis to the minor axis (acicular ratio) of said iron oxidepowder being from 1.3 to 4.5 and the coercive force of said iron oxidepowder being from 550 to 800 Oe.
 2. The magnetic recording mediumaccording to claim 1, wherein said average grain size of saidcobalt-adsorbed iron oxide powder is between 0.05 micron to 0.5 micron.3. The magnetic recording medium according to claim 1, wherein saidaverage grain size of said cobalt-adsorbed iron oxide powder is between0.1 micron to 0.3 micron.
 4. The magnetic recording medium according toclaim 1, wherein said support is selected from the group consisting ofpolymeric resins, non-magnetic metals and ceramics.
 5. The magneticrecording medium according to claim 4, wherein said polymeric resin isselected from the group consisting of polyesters, polyolfins, cellulosederivatives, and plastics selected from polycarbonate,polyvinylchloride, polyimide and nylon.
 6. The magnetic recording mediumaccording to claim 4, wherein said support is a non-magnetic metalsupport of aluminium, zinc or an alloy thereof.
 7. The magneticrecording medium according to claim 4, wherein said support is in theform of film or sheet, card, disc or cylindrical drum.
 8. The magneticrecording medium according to claim 7, wherein the proportion of thecobalt adsorbed is 0.5 to 30% by weight relative to 100% by weight ofsaid iron oxide.
 9. The magnetic recording medium according to claim 7,wherein the proportion of the cobalt adsorbed is 2.5 to 30% by weightrelative to 100% by weight of said iron oxide.
 10. The magneticrecording medium according to claim 1, wherein said magnetic layercontains a binder.
 11. The magnetic recording medium according to claim10, wherein said binder is contained in said magnetic layer at aproportion of 5 to 400 parts by weight relative to 100 parts by weightof of said iron oxide powder.
 12. The magnetic recording mediumaccording to claim 10, wherein said binder is contained in said magneticlayer at a proportion of 10 to 100 parts by weight relative to 100 partsby weight of of said iron oxide powder.
 13. The magnetic recordingmedium according to claim 10, wherein said binder is selected from thegroup consisting of thermoplastic resins, thermosetting resins,radiation curable resins, reactive resins, electron-irradiation-settingresins and any mixture thereof.
 14. The magnetic recording mediumaccording to claim 9, wherein said magnetic layer also contains a binderin an amount of between 10 and 100 parts by weight per 100 parts byweight of said ferromagnetic powder and wherein said average grain sizeof said powder is between 0.05 micron and 0.5 micron.
 15. The magneticrecording medium according to claim 14, wherein said binder is selectedfrom the group consisting of thermoplastic resins, thermosetting resins,radiation curable resins, reactive resins, electron-irradiation-settingresins and mixtures thereof and wherein said support is a polymericmaterial selected from the group consisting of polyesters, polyolefins,cellulose derivatives, polycarbonate, polyvinylchloride, polyimide andnylon.
 16. The magnetic recording medium according to claim 15, whereinsaid average grain size of said iron oxide powder is between 0.1 micronto 0.3 micron.
 17. The magnetic recording medium according to claim 15,wherein said support is a non-magnetic metal support of aluminum, zincor an alloy thereof and wherein said average grain size of said powderis between 0.1 micron to 0.3 micron.
 18. The magnetic recording mediumaccording to claim 1, wherein said cobalt was adsorbed on the surface ofsaid iron oxide powder after said iron oxide was formed as particleswhich comprise said iron oxide powder.
 19. The magnetic recording mediumaccording to claim 1, wherein said iron oxide is γ-Fe₂ O₃ having cobaltadsorbed on the surface thereof.